The tensor power spectrum is obtained by solving the equation of motion for the tensor perturbation which is given by the transverse-traceless component of the second-order Einstein equation. The equation of motion is given, in the Fourier space, by [1]}, [2]}, [3]}
\(h_{\bf k}^{\prime \prime } + 2\mathcal {H}h_{\bf k}^\prime + k^2h_{\bf k} = 4S_{\bf k}\,,\) 
